The invention relates to an electrochromic display device having display segments which are individually connected to a power source and whose electric leads are insulated from the electrochromic medium. The invention also relates to a method of producing such a display device.
Electrochromic devices containing an electrochromic system are already known. Electrochromic systems alter their spectral absorption under the action of an electric field.
In WO-A 94/23333, electrochromic materials of different constructions are compared but are not used as display devices:
Construction a: The electrochromic substances are present in solid form as a film or layer on the electrodes (cf. D. Theis in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A8, p. 622, Verlag Chemie 1987)
Construction b: The electrochromic substances are deposited on the electrodes as a layer during the redox process (cf. Ullmann, see above).
Construction c: The electrochromic substances remain permanently in solution.
For construction a), the pair tungsten oxide/palladium hydride is the best known electrochromic material.
For construction b), viologens have been described as electrochromic substances. These devices are not self-extinguishing, so the image produced remains after the electric potential has been switched off and can be extinguished again only by reversing the polarity. Such devices are not particularly stable and do not allow a large number of switching cycles.
In addition, particularly cells based on tungsten oxide/palladium hydride cannot be operated in transmitted light because of light scattering from these electrochromic layers, but can only be operated using reflected light.
Elektrokhimiya, 13, 32-37 (1977), 13, 404-408, 14, 319-322 (1978), U.S. Pat. No. 4,902,108 and U.S. Pat. No. 5,140,455 disclose an electrochromic system having the last-named construction c). An electrochromic cell which is built up of glass plates having a conductive coating contains a solution of a pair of electrochromic substances in an inert solvent.
The pair of electrochromic substances used comprises one electrochemically reversibly reducible substance and one reversibly oxidizable substance. In their ground state, both are colourless or only slightly coloured. Under the action of an electric potential, one substance is reduced and the other is oxidized, with both becoming coloured. After switching off the potential both substances return to the ground state, resulting in decolouration or lightening of the colour. 
It is known from U.S. Pat. No. 4,902,108 that pairs of redox substances in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves are suitable.
However, according to WO-A 94/23333, such solution systems of construction c) have serious disadvantages.
The diffusion of the electrochromic substances in the solution results in fuzzy colour boundaries and causes high power consumption for maintaining the coloured state, since the coloured substances are permanently being removed by recombination and reaction at the respective opposite electrode.
Nevertheless, various applications have been described for such electrochromic cells of the construction c). Thus, for example, they can be configured as an automobile rearview mirror which can be dimmed during night driving by application of a voltage and thus prevents dazzling by headlights of following vehicles (cf., for example, U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A 0 435 689). Furthermore, such cells can also be used in window panes or automobile sunroofs where they dim sunlight after application of a voltage. The use of such devices as electrochromic display devices, for example in segmented or matrix displays with structured electrodes, has likewise been described (DE 196 31 728).
Electrochromic devices comprise a pair of glass or plastic plates which are each provided on one side with an electrically conductive coating, e.g. indium-tin oxide (ITO). At least one of these plates is transparent. In the case of an automobile mirror, one of the plates is mirrored. A cell is constructed from these plates by joining them, preferably adhesively bonding them, by means of a circular or rectangular sealing ring with their conductively coated sides facing one another. The sealing ring establishes a uniform spacing between the plates of, for example, from 0.01 to 0.5 mm. This cell is filled with an electrochromic medium. The two plates can be separately connected to a power source via the electrically conductive layers.
In electrochromic display devices, at least one of the two electrically conductive layers is divided into segments which are individually connected to a power source and are electrically insulated from one another. These segments are provided with power via electric leads which each lead to an edge of the respective plate and are there connected to a power source by means of, for example, clips, soldered points, conductive paint or other electrically conductive connections.
During operation of the electrochromic device, an electric potential between the conductively coated plates is applied to selected segments, so that the desired information is displayed by colouration of the electrochromic medium. However, the potential is likewise present at the power leads, as a result of which an electrochromic reaction also takes place along these and the leads likewise become coloured. This is undesirable since the image defined by the segments is considerably disrupted thereby.
It was an object of the invention to produce an electrochromic display device having segments which are individually connected to a power source, in which device the power leads to the segments to which a potential is being applied do not cause a colour reaction.
The object of the invention is achieved by an electrochromic display device comprising a pair of glass or plastic plates or plastic films of which at least one plate or film, preferably both plates or films, is/are provided on one side each with an electrically conductive coating and of which at least one plate or film and its conductive coating is transparent and of which the other can be mirrored and in which the electrically conductive layer of at least one of the two plates or films is divided into separate segments which are individually connected to a power source and each has an electric lead from one edge of the respective plate or film, where the plates or films are joined via a sealing ring on the sides on which they are conductively coated and the volume formed by the two plates or films and the sealing ring is filled with an electrochromic medium, characterized in that the electric leads to the segments are electrically insulated from the electrochromic medium. Due to different demands upon the electrochromic display device the division into seperate segments can be carried out differently. If only one sign or symbol shall be displayed, only one segment is prepared. Alternatively several segments can be prepared. The shape and number of the said several segments can be chosen, so that with the combination of these segments all kinds of signs like letters, figures, symbols, words, numbers and combinations thereof can be displayed. The shape of a single segment can be that of the sign itself or that of a rectangle, square, multiangle, circle, oval and/or other geometric object or combinations thereof.
The insulation can be achieved, for example, by the electric leads to the areas being coated with an electrically insulating layer. Preferably said electrically insulating layer is transparent.
As electrically conductive coatings on the glass plates or plastic plates or plastic films from which the electrochromic display device is constructed, it is in principle possible to use any electrically conductive materials. However, a transparent plate or film has to have a conductive coating which is itself transparent. Suitable, preferably transparent, materials are, for example, indium-tin oxide (ITO), antimony- or fluorine-doped tin oxide, antimony- or aluminium-doped zinc oxide, zinc oxide or conductive organic polymers such as substituted or unsubstituted polythienyls, polypyrroles, polyanilines, polyacetylene. In this way, a transmissive electrochromic device which can be viewed in transmitted light is obtained.
One of the plates or films can, however, also have a metallic, preferably opaque, electrically conductive coating. Suitable materials are copper, silver, gold, chromium, aluminium, palladium or rhodium or else palladium on chromium or rhodium on chromium or other metal combinations. In this way, a reflective electrochromic device is generally obtained.
To achieve an electrochromic display device, which reflects uniformly it is advantageous to coat the plates with a series of layers. The plate or film is coated uniformly with a mirroring material such as one of the above mentioned metals or combinations thereof. On this mirroring layer an electrically insulating layer, such as one of the below mentioned, is coated. On this electrically insulating layer another metallic layer comprising one of the above mentioned metals or combinations thereof is coated. From this metallic layer the segments and the electric leads are being formed. The production of the segments and the electric leads will be described below. The electric leads are coated with an electrically insulating layer as described below.
The mirroring materials can also be applied to the nonconductive, uncoated side of one of the above-described transparent and transparently conductively coated plates or films. In this case, the metal layer serves not as a conductor but only as a reflector. This also gives a reflective electrochromic device.
However, these materials can also be used in mixed form, e.g. a grid made of a metallic conductor which is, for example, coated with an ITO layer.
A preferred embodiment of the electrochromic display device of the invention is one in which the segments and the electric leads consist of the same electrically conductive material.
Such materials can, for example, be any of the abovementioned transparent and opaque conductive materials, e.g. ITO or gold, chromium or mixed materials such as the combination of a metallic grid with, for example, ITO.
A likewise preferred embodiment of the electrochromic display device of the invention is one in which the segments and the electric leads consist of different electrically conductive material.
In this case, the segments consist, for example, of one of the abovementioned transparent materials, e.g. ITO, and the leads consist of a material having a particularly good conductivity. The leads can be metallic leads, e.g. gold or palladium, or combined leads of a material such as ITO and a metallic conductor, e.g. gold, in the form of wires or grids.
The electric resistance of the electric leads is preferably as low as possible. Only if the electric resistance is low the intensity of colouring of the segments does not vary with the length of the electric leads to the segment. Preferred materials for electric leads are therefore metals or combinations of metals with another electrically conductive material. Also suitable, although not comprising a metal, are electric leads made out of ITO, if they have a big width.
The segments and their leads are first produced by removal of material from a continuous conductive coating on glass or plastic plates or films or are applied to the glass or plastic plates or films. The electrically insulating layer is subsequently applied to the leads.
The production of the segments and leads by removal of material can be carried out by mechanical removal of the conductive layer, for example by scoring, scratching, scraping or milling, or by chemical means, for example by etching using, for example, a hydrochloric acid solution of FeCl2 and SnCl2 or other agents such as HNO3. The places at which the conductive layer is removed in this way can be controlled by means of masks, e.g. masks of photoresist. However, the electrically separate segments and leads can also be produced by targeted, e.g. by means of masks, application of the conductive layer, e.g. by sputtering or printing. All the techniques which have just been described are generally known from the production of liquid crystal displays (LCDs).
Another method to produce the segments and the electric leads and to shape the segments comprises the step of coating the electrically conductive layer with the electrically insulating layer. If only one segment is to be displayed, then an electrically isolating layer is coated as described below on an uniform conductive layer, so that it does not cover the segment, but all other parts of the conductive layer. If the segment is of the shape of a symbol, then the coating covers the conductive layer despite a part of the shape of that symbol.
The advantage of this method is the simplicity of the production of the segments, since a structuring of the conductive layer is avoided. Another advantage is, that no electric leads, which might have a high resistance are needed, but the whole conductive layer leads the current, so that the segments colour uniformly. This method allows to produce a segment, which might comprise several parts e.g. several letters for example via a mask after a model of the desired symbol. Changes in the design of the segments can be performed very easy, just by changing the shapes of the recesses in the electrically insulating layer. Even if the symbols have a complex shape and are fine-structured, a uniform colouring takes place.
If the electrochromic display device comprises several segments, which are to be switched seperately, then the conductive layer is devided into big parts with small strips in between, for example by the method of etching. Each part should be big enough to contain al least one segment and its electric connections. All these parts are being coated simultanously with an electrically isolating layer, so the segements, which are to be displayed, are not covered (see FIG. 5).
If an electrochromic display device comprises several segments, it is not always possible to find enough space to lead all the electric leads to the edge of the respective plate or film. In such a case it is preferable to contact the segments through the plate or the film. In the vicinity of the segment, which is to be contacted, the plate or film comprises a little hole through which the electric leads are connected to the electric conductive layer. The diameter of the small hole is preferably 0, 2 to 1 mm. The holes are produced by drilling, punching or etching, depending on the material of the plate or film.
Through the hole in the plate or film the electric lead is connected with the segment. At the back of the plate or film the electric leads are connected in conventional manner with wires or printed circuits. The diameter of the external cables can be chosen as large as necessary to avoid potential differences without affecting the design of the segments in the electrochromic display device.
The connection between a segment and the lead conducted trough the hole can be performed in different manners, e.g. with conducting varnish. In a preferred embodiment the inner part of the hole is coated with a conductive material and directly connected to the segment. To stabilize such a connection through the hole mechanically the wire at the back of the plate or film can be pasted over with the conductive varnish and the plate or film in the vicinity of the hole by an adhesive (see FIG. 7).
This type of electric connection allows to position the segments very close to each other, since no space is needed to lead the electric leads to the edge of the plate or film.
As electrically insulating layer according to the invention, use is made of an organic, macromolecular compound or an inorganic compound. Preferred organic compounds are adhesives and varnishes/paints.
The electrochromic media known from the prior art contain pairs of redox substances which, after reduction or oxidation, form coloured free radicals, radical cations or radical anions which are chemically reactive. As is known, for example, from Topics in Current Chemistry, Vol. 92, p. 1-44 (1980), such free radicals or radical ions can be sensitive to electrophiles or nucleophiles or other free radicals. For this reason, if an electrochromic device containing such an electrochromic medium is to have a high stability and survive several thousand switching cycles, it has to be ensured that the electrochromic medium is absolutely free of electrophiles, e.g. protons, nucleophiles and oxygen. Furthermore, it has to be ensured that such reactive species are not formed at the electrodes by electrochemical processes during operation of the electrochromic device.
However, the electrically insulating layer on the electric leads of the electrochromic cell has to contain, release or, when in contact with the electrochromic system, form no reactive constituents which react with the electrochromic substances or the species formed from them electrochemically, e.g. the abovementioned radical or radical ion species or the doubly reduced or doubly oxidized species which are formed electrochemically or are always present in equilibrium.
Suitable adhesives and varnishes/paints are epoxy and acrylate systems.
Epoxy adhesives are known, for example, from J. W. Muskopf, S. B. McCollister in Ullmann""s Encyclopedia of Organic Chemistry, VCH Verlagsgesellschaft mbH, 5th edition, Vol. A 9, p. 547 ff, 1987. Curing can occur by an anionic or cationic mechanism.
Acrylic adhesives are known, for example from W. Dierichs et al. in Ullmann""s Encyklopxc3xa4die der technischen Chemie, VCH Verlagsgesellschaft mbH, 4th edition, Volume 14, p. 233 ff, 1977. Curing occurs by a free-radical mechanism which can be initiated by means of ultraviolet radiation with the aid of added photoinitiators.
The epoxy adhesive used according to the invention for the electrical insulation of the leads is a two-component or one-component adhesive. It is preferably a two-component epoxy adhesive in which, for example, one component contains an epoxide compound and the other component contains an amine, an anhydride or a Lewis acid or Lewis base or is a one-component epoxy adhesive which contains, for example, an epoxide compound and a capped Lewis or Brxc3x8nsted acid or Lewis or Brxc3x8nsted base which under the action of heat or light sets free a Lewis or Brxc3x8nsted acid or Lewis or Brxc3x8nsted base.
The epoxide component is preferably an epoxide of the formula 
where
V represents a bridge which may likewise bear epoxy groups, and the amine component is a primary or secondary aliphatic, cycloaliphatic, aromatic, araliphatic or heterocyclic, at least bifunctional amine and the Lewis acid which is present in capped form is a nonmetal halide and the Brxc3x8nsted acid which is present in capped form is a strong protic acid which is derived from a nonmetal halide.
Such nonmetal halides are, for example, boron trifluoride, boron trichloride, phosphorus pentafluoride, arsenic pentafluoride, arsenic pentachloride, antimony pentafluoride or antimony pentachloride. The protic acids derived therefrom are then, for example, HBF4, HBCl4, HPF6, HAsF6, HAsCl6, HSbF6, HSbCl6. Capped nonmetal halides are, for example, adducts of amines with these nonmetal halides, for example BF3.NH(C2H5)2, BF3.NH(CH3)C2H, etc. Such compounds dissociate on heating to release the nonmetal halides. Tetrafluoroborates of aromatic or heterocyclic diazonium salts are also capped nonmetal halides. They dissociate photochemically to release BF3. Diaryliodonium or triarylsulphonium salts containing anions of the abovementioned protic acids, e.g. (C6H5)2I+PF6xe2x88x92 dissociate photochemically to form the corresponding protic acids, e.g. HPF6. Such capped products are known from, for example, R. S. Bauer in R. W. Tess, G. W. Poehlein (eds.) Applied Polymer Science, 2nd ed., ACS Symposium Series 285, ACS Washington, 1985, p. 931-961.
These adhesives can additionally contain other components such as glycidyl esters or glycidyl ethers, phenols or alcohols and, in the case of the photochemically curable adhesives, peroxides or ferrocene. Examples are glycidyl neodecanoate, hexanediol glycidyl ether, phenol, benzyl alcohol and cumene hydroperoxide.
Acrylate adhesives used according to the invention for the electrical insulation of the leads preferably comprise a bisacrylate and a trisacrylate component into which there is mixed a photoinitiator which liberates free radicals under the action of light in the UV region or the neighbouring blue spectral region.
The epoxide component is particularly preferably an epoxide of the formula 
where
V represents a bridge which can likewise bear epoxy groups,
and the Lewis acid which is present in capped form is a nonmetal halide and the Brxc3x8nsted acid which is present in capped form is a strong protic acid derived from a nonmetal halide.
Lewis and Brxc3x8nsted acids and their capped forms are described in more detail above.
Likewise particularly preferably, the acrylate mixture comprises compounds of the formulae 
where
n represents an integer from 0 to 20, preferably from 0 to 10 and
R101 represents hydrogen or methyl and 
xe2x80x83where
R102 represents hydrogen or methyl.
The composition of the acrylate mixture of (CCI) and (CCII) is particularly preferably in the range (parts of CCI)/(parts of CCII) 1 to 5.
The photoinitiators are particularly preferably compounds based on benzophenone derivatives or thioxanthones which produce free radicals on photolysis. Photoinitiators are particularly preferably used for UV curing, for example Irgacure(copyright) 651 (Ciba-Geigy), Darocur(copyright) 1116, Darocur(copyright) 1173, Darocur(copyright) 1664, Darocur(copyright) 2273, Darocur(copyright) 4043 (all from E. Merck, Darmstadt). The particularly preferred concentration range of these photoinitiators is from 0.01 to 5% by weight.
The epoxide component of the epoxy adhesive used according to the invention is very particularly preferably an epoxide of the formula 
where
m and p each represent, independently of one another, an integer from 0 to 20, preferably from 0 to 5, and
R100 represents hydrogen or methyl
and the amine component is an aliphatic polyamine of the formula 
xe2x80x83where
q represents an integer from 2 to 10, preferably from 2 to 4, or
an araliphatic amine such as xcex1,xcex1xe2x80x2-diamino-m- or -p-xylene or polyaminoimidazoline or
an aromatic amine such as bis-(4-aminophenyl)methane or bis-(4-aminophenyl)) sulphone
and the capped Lewis acid is an adduct of boron trifluoride and a secondary amine, e.g. BF3xe2x80x94NH(CH3)C2H5 or a diazonium salt such as Arxe2x80x94N2I+BF4xe2x88x92 or the capped Brxc3x8nsted acid is an iodonium salt such as (Ar)2I+BF4xe2x88x92 or (Ar)2Ixe2x88x92PF6xe2x88x92 or a sulphonium salt such as (Ar)3S+BF4xe2x88x92 or (Ar)3S+PF6xe2x88x92,
xe2x80x83where
Ar represents an aromatic radical, preferably phenyl.
Likewise very particularly preferably, the acrylates comprise a mixture of compounds of the formulae 
where
n represents an integer from 0 to 20, preferably from 5 to 15, and
R101 represents hydrogen or methyl and 
xe2x80x83where
R102 represents hydrogen or methyl.
The composition of the acrylate mixture of (CCI) and (CCII) is very particularly preferably in the range (parts of CII)/(parts of CCII)=1 to 2.
The photoinitiators are very particularly preferably compounds based on benzophenone derivatives or thioxanthones which produce free radicals on photolysis. Very particular preference is given to using photoinitiators for UV curing, e.g. very particularly Darocur(copyright) 1173 (E. Merck, Darmnstadt) and Irgacure(copyright) 651 (Ciba-Geigy). The very particularly preferred concentration range of these photoinitiators is from 0.1 to 3% by weight.
Most particularly preferred thermally curing epoxy adhesives contain epoxides of the formula (CCIII),
where
m represents from 2 to 5,
and amines of the formula (CCV),
xe2x80x83where
q represents from 2 to 4,
and xcex1,xcex1xe2x80x2-diamino-m-xylene or mixtures thereof and, if desired, further epoxide compounds such as glycidyl esters, for example, glycidyl neodecanoate, and/or glycidyl ethers, for example hexanediol glycidyl ether.
Most particularly preferred epoxy adhesives which cure photochemically or with photochemical initation contain epoxides of the formula (CCIII),
where
m represents from 2 to 5,
and a diazonium salt such as Arxe2x80x94N2+BF4xe2x88x92 as capped Lewis acid or an iodonium salt such as (Ar)2+BF4xe2x88x92 or (Ar)2I+PF6or a sulphonium salt such as (Ar)3S+BF4xe2x88x92 or (Ar)3S+ PF6as capped Brxc3x8nsted acid,
xe2x80x83where
Ar represents an aromatic radical, preferably phenyl.
Most particularly preferred photochemically curing acrylate adhesives contain acrylates of the formulae (CCI)
where
represents from 5 to 10 and
R101 represents hydrogen and
and (CCII)
xe2x80x83where
R102 represents hydrogen.
The most particularly preferred composition of the acrylate mixture of (CCI) and (CCII) is (parts of CCI)/(parts of CCII)=5/3.
The most particularly preferred photoinitiator Darocur(copyright) 1173 (E. Merck, Darmstadt) is used in a concentration of from 0.5% by weight.
The curing of the adhesives depends on their chemical composition.
Thermally curable epoxy adhesives based on epoxides of the formula (CCIII),
where
m represents from 2 to 5,
and amines of the formula (CCV),
xe2x80x83where
q represents from 2 to 4,
and xcex1,xcex1xe2x80x2-diamino-m-xylene or mixtures thereof and, if desired, further epoxide compounds such as glycidyl esters, for example glycidyl neodecanoate, are cured, for example, at temperatures of from 90 to 170xc2x0 C., preferably from 110 to 150xc2x0 C., over a period of, for example, from 5 to 60 minutes, preferably from 10 to 30 minutes.
Epoxy adhesives which cure photochemically or with photochemical initiation and are based on epoxides of the formula (CCIII),
where
m represents from 2 to 5,
and a capped Lewis acid, for example NO2xe2x80x94C6H4xe2x80x94N2+BF4xe2x88x92, or a capped Brxc3x8nsted acid, for example (C6H5)2Ixe2x88x92BF4xe2x88x92 or (C6H5)2I+PF6xe2x88x92 or (C6H5)3S+BF4xe2x88x92 or (C6H5)3S+PF6xe2x88x92 are either completely cured by means of visible or UV light or, more advantageously, merely initiated by means of visible or UV light, without fully curing the adhesive, and then fully cured at room temperature or elevated temperature, for example at from 30 to 150xc2x0 C., preferably from 70 to 130xc2x0 C. The curing time depends on the temperature. Thus, for example, it can be from 10 to 24 hours at room temperature but only from 10 to 30 minutes at 110xc2x0 C. Illumination can be carried out using a UV lamp, a flashlamp or possibly daylight or the light of daylight-type lamps.
In adhesives which have been cured in this way, the constituents of the adhesive, in particular the epoxide and amine constituents, are incorporated so firmly and completely into the macromolecular structure that they can no longer react with the electrochromic medium used according to the invention or the species formed by reduction or oxidation. The cured adhesives are also so stable that they withstand thermal stressing, for example at from xe2x88x9240 to +150xc2x0 C., without problems and, in this temperature range, are also not dissolved, swollen or permeated by the electrochromic liquid used according to the invention.
Photochemically curable acrylate adhesives based on acrylates of the formulae (CCI)
where
n represents from 5 to 15 and
R101 represents hydrogen and
and (CCII)
xe2x80x83where
R102 represents hydrogen
having a composition of the acrylate mixture (parts of CCI)/(parts of CCII)=5/3 and a concentration of Darocur(copyright) 1173 photoinitiator of 0.5% by weight are fully cured at room temperature by means of UV light from a suitable lamp.
Adhesives which have been cured in this way withstand thermal stressing, for example at from xe2x88x9240 to +105xc2x0 C., without problems and, in this temperature range, are also not dissolved, swollen or permeated by the electrochromic liquid used according to the invention.
The above-described adhesives are applied to the electric leads, for example by doctor blade coating or printing, and then cured as described above. However, the areas of the plates or films which do not have a conductive coating can also be covered partly or completely with this adhesive.
In the case of the photochemically curable adhesives, a different procedure can also be employed: they are applied to the entire area of the sides of the plates or films which have been conductively coated and divided into segments and electric leads. The electric leads are subsequently illuminated through a mask so that the adhesive cures at these places. The adhesive which has not been cured is subsequently removed from the unilluminated places, e.g. by means of a suitable solvent. In this procedure too, the areas of the plates or films which have not been conductively coated can be partly or completely illuminated and thus covered with cured adhesive.
It is also possible to use an inorganic compound as electrically insulating layer. Such inorganic layers are applied to the electric leads, for example by printing, sputtering or similar techniques. They can also be produced from precursors which have been applied to the leads, for example by thermolysis or oxidation.
Suitable inorganic insulating compounds are oxides, for example silicon oxide. Such oxides are applied, for example, by sputtering. Typical thicknesses of such oxide layers are, for example, from 10 to 5000 nm, preferably from 20 to 1000 nm.
If the electric leads are made from metal, then the electrically insulating layer can be an oxide of that metal. Preferably it is produced by oxidation of the surface of the metallic electric lead. The segments, especially if the are made of metal are covered by an adhesive film, by a photoresist or any other tight mask during the process of oxidation. The thickness of the oxide layer produced by the oxidation process should be thick enough to be insulating. The oxide layer should not contain any pinholes. An example for this kind of insulation is a conductive layer made of aluminum and an insulating layer made of aluminum oxide.
The electrochromic medium in the electrochromic device of the invention contains at least one pair of redox substances OX2 and REd1, of which one is reducible and the other is oxidizable and both are colourless or only slightly coloured and after application of an electric potential to the electrochromic device one substance is reduced and the other is oxidized so that at least one becomes coloured and after switching off the electric potential the two original redox substances are formed again and the electrochromic device loses its colour.
However, the reverse case can also apply, namely where at least one of the two electrochromic sustances is coloured and becomes colourless or changes colour as a result of reduction or oxidation at the cathode or anode respectively.
Suitable electrochromic substances are known, e.g. from D. Theis in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A8, p. 622, Verlag Chemie 1987, U.S. Pat. No. 4,902,108, WO 97/30 134 and 97/30 135.
Preference is given to electrochromic media in which the electrochromic substances are present in solution, in gels or in solid layers; particular preference is given to solutions and gels.
Preference is given to electrochromic devices according to the invention in which
a) the reducible substance has at least one, preferably at least two, chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least one, preferably at least two, chemically reversible oxidation waves, or
b) the reducible substance and the oxidizable substance are covalently bound to one another via a bridge B, or
c) the reducible and/or oxidizable substances selected are ones in which the reversible transition between the oxidizable form and the reducible form or vice versa is associated with the rupture or formation of a "sgr" bond, or
d) the reducible substance and/or the oxidizable substance are metal salts or metal complexes of metals which exist in at least two oxidation states, or
e) the reducible and/or oxidizable substances are oligomers and polymers which contain at least one of the redox systems mentioned, or else pairs of redox systems as defined under a) to d), or
f) the reducible and/or oxidizable substances used are mixtures of the substances described in a) to e), provided that these mixtures contain at least one reducible and at least one oxidizable redox system.
Selection of the electrochromic compounds RED1 and OX2 and/or mixtures thereof enables any monochrome colours to be produced. For a polychrome image, two or more such electrochromic devices can be laid on top of one another as layers, with each of these devices being able to produce a different colour. Such a stack is preferably built up in such a way that the devices which are in contact have a common translucent plate which is thus conductively coated on both sides and, depending on the design, is divided into segments. For example, a stack then comprises three electrochromic devices incorporating at least four plates. Switching on segments in various devices in this stack enables multicoloured displays to be achieved. If superimposed segments of various devices in such a stack are switched on, mixed colours are obtained. Thus, combination of three appropriate colours enables any colours to be produced, for example colour pictures.
Compounds OX2 and RED1 which are suitable for the purposes of the invention are substances which on reduction or oxidation at the cathode or anode respectively in the appropriate solvent give products RED2 and OX1 which undergo no subsequent chemical reaction but can be reoxidized or rereduced intact to form OX2 and RED1 again.
Suitable reducible substances OX2 are, for example, 
where
R2 to R5, R8, R9, R16 to R19 are, independently of one another, C1- to C18-alkyl, C2- to C12-alkenyl, C4- to C7-cycloalkyl, C7- to C15-aralkyl or C6- to C10-aryl or
R4; R5 or R8; R9 may together form a xe2x80x94(CH2)2xe2x80x94 or xe2x80x94(CH2)3xe2x80x94 bridge,
R6, R7 and R22 to R25 are, independently of one another, hydrogen, C1- to C4-alkyl, C1- to C4-alkoxy, halogen, cyano, nitro or C1- to C4-alkoxycarbonyl or
R22; R23 and/or R24; R25 may form a xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 bridge,
R10; R11, R10; R13, R12; R13 and R14; R15 are, independently of one another, hydrogen or, in pairs, a xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)3xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 bridge,
R20 and R21 are, independently of one another, O, Nxe2x80x94CN, C(CN)2 or Nxe2x80x94C6- to C10-aryl,
R26 and R27 are hydrogen, C1- to C4-alkyl, C1- to C4-alkoxy, halogen, cyano, nitro, C1- to C4-alkoxycarbonyl or C6- to C10-aryl,
R69 to R74, R80 and R81 are, independently of one another, hydrogen or C1- to C6-alkyl or
R69; R 12, R70; R13, R73; R80 and/or R74; R1 together form a xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 bridge,
E1 and E2 are, independently of one another, O, S, NR1 or C(CH3)2 or
E1 and E2 together form a xe2x80x94Nxe2x80x94(CH2)2xe2x80x94Nxe2x80x94 bridge,
R1 is C1- to C18-alkyl, C2- to C12-alkenyl, C4- to C7-cycloalkyl, C7- to C15-aralkyl, C6- to C10-aryl,
Z1 is a direct bond, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94C(CH3)xe2x95x90CHxe2x80x94, xe2x80x94C(CN)xe2x95x90CHxe2x80x94, xe2x80x94CClxe2x95x90CClxe2x80x94, xe2x80x94C(OH)xe2x95x90CHxe2x80x94, xe2x80x94CClxe2x95x90CHxe2x80x94, xe2x80x94C Cxe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94Nxe2x95x90CHxe2x80x94, xe2x80x94C(CH3)xe2x95x90Nxe2x80x94Nxe2x95x90C(CH3)xe2x80x94 or xe2x80x94CClxe2x95x90Nxe2x80x94Nxe2x95x90CClxe2x80x94,
Z2 is xe2x80x94(CH2)rxe2x80x94 or xe2x80x94CH2xe2x80x94C6H4xe2x80x94CH2xe2x80x94,
r is an integer from 1 to 10,
R101 to R105 are each, independently of one another, C6- to C10-aryl or an aromatic or pseudoaromatic, five- or six-membered heterocyclic ring which may optionally be benzo-fused,
R107, R109, R113 and R114 are each, independently of one another, a radical of the formulae (CV) to (CVII) 
R108, R115 and R116 are each, independently of one another, C6- to C10-aryl or a radical of the formula (CV),
R110 to R112, R117 and R118 are, independently of one another, hydrogen, C1- to C4-alkyl, halogen or cyano,
E100 and E102 are, independently of one another, O, S or Nxe2x80x94R119,
R119 and R122 are, independently of one another, C1- to C18-alkyl, C2- to C8-alkenyl, C4- to C7-cycloalkyl, C7- to C15-aralkyl or C6- to C10-aryl,
R106, R120, R121, R123 and R124 are, independently of one another, hydrogen, C1- to C4-alkyl, C1- to C4-alkoxy, halogen, cyano, nitro or C1- to C4-alkoxycarbonyl or
R120, R121 or R123, R124 together form a xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 bridge and
Xxe2x88x92 is an anion which is redox-inert under the conditions.
Suitable oxidizable substances RED1 are, for example, 
where
R28 to R31, R34, R35, R38, R39, R46, R53, and R54 are, independently of one another, C1- to C18-alkyl, C2- to C12-alkenyl, C4- to C7-cycloalkyl, A7- to C15-aralkyl or C6- to C10-aryl,
R32 to R33, R36, R37, R40, R41, R42 to R45, R47, R48, R49 to R52 and R55 to R58 are independently of one another, hydrogen, C1- to C4-alkyl, C1- to C4-alkoxy, halogen, cyano, nitro, C1- to C4-alkoxycarbonyl, C6- to C10-aryl and
R57 and R58 may also each be an aromatic or pseudoaromatic, five- or six-membered heterocyclic ring which may optionally be benzo-fused and R48 may also be NR75R76 or
R49; R50 and/or R51; R52 form a xe2x80x94(CH2)3, xe2x80x94(CH2)4xe2x80x94, xe2x80x94(CH2)5xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 bridge,
Z3 is a direct bond, a xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Nxe2x95x90Nxe2x80x94 bridge,
xe2x95x90Z4xe2x95x90 is a direct double bond, a xe2x95x90CHxe2x80x94CHxe2x95x90 or xe2x80x94Nxe2x80x94Nxe2x95x90 bridge,
E3 to E5, E10 and E11 are, independently of one another, O, S, NR59 or C(CH3)2 and
E5 may also be Cxe2x95x90O or SO2,
E3 and E4 may also be, independently of one another, xe2x80x94CHxe2x95x90CHxe2x80x94,
E6 to E9 are, independently of one another, S, Se or NR59,
R59; R75 and R76 are, independently of one another, C1- to C12-alkyl, C2- to C8-alkenyl, C4- to C7-cycloalkyl, C7- to C15-aralkyl, C6- to C10-aryl, and
R75 may also be hydrogen or R75 and R76 in NR75R76 may also, together with the N atom to which they are bound, represent a five- or six-membered ring which may optionally contain further heteroatoms,
R61 to R68 are, independently of one another, hydrogen, C1- to C6-alkyl, C1- to C4-alkoxy, cyano, C1- to C4-alkoxycarbonyl or C6- to C10-aryl and
R61; R62 and R67; R68 may also, independently of one another, form a xe2x80x94(CH2)3xe2x80x94, xe2x80x94(CH2)4xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 bridge, and
R62; R63, R64; R65 and R66; R67 form a xe2x80x94Oxe2x80x94CH2CH2xe2x80x94Oxe2x80x94 or a xe2x80x94Oxe2x80x94CH2CH2CH2xe2x80x94Oxe2x80x94bridge, and
v is an integer from 0 to 10.000.
Likewise suitable as RED1 are anions such as Ixe2x88x92, I3xe2x88x92, Brxe2x88x92, SCNxe2x88x92.
Examples of redox systems which are linked via a bridge B and may be oligomeric or polymeric are those of the formula
Yxe2x80x94[xe2x80x94(xe2x80x94Bxe2x80x94Zxe2x80x94)axe2x80x94(xe2x80x94Bxe2x80x94Yxe2x80x94)bxe2x80x94]cxe2x80x94Bxe2x80x94Zxe2x80x83xe2x80x83(I),
where
Y and Z each represent, independently of one another, a radical OX2 or RED1, but at least one Y represents OX2 and at least one Z represents REd1,
xe2x80x83where
OX2 represents the radical of a reversibly electrochemically reducible redox system, and
RED1 represents the radical of a reversibly electrochemically oxidizable redox system,
B represents a bridge,
c represents an integer from 0 to 1000, and
a and b each represent, independently of one another, an integer from 0 to 100.
(a+b)*c is preferably xe2x89xa610,000.
Here, the term reversibly electrochemically reducible or oxidizable means that electron transfer can occur in the sense of the above definition of OX2 and RED1 used according to the invention, either with or without a change in the "sgr" skeleton.
In particular, the electrochromic compounds of the formula (I) are those of the formulae
xe2x80x83OX2xe2x80x94Bxe2x80x94RED1xe2x80x83xe2x80x83(Ia),
OX2xe2x80x94Bxe2x80x94RED1xe2x80x94Bxe2x80x94OX2xe2x80x83xe2x80x83(Ib),
RED1xe2x80x94Bxe2x80x94OX2xe2x80x94Bxe2x80x94RED1xe2x80x83xe2x80x83(Ic), or
OX2xe2x80x94(Bxe2x80x94RED1xe2x80x94Bxe2x80x94OX2)dxe2x80x94Bxe2x80x94RED1xe2x80x83xe2x80x83(Id)
where
OX2, RED1 and B are as defined above and
d represents an integer from 1 to 5.
OX2 and RED1 in the formulae (I) and (Ia) to (Id) are in particular radicals of the above-described redox systems of the formulae (II), (IIa) to (IX), (CI) to (CIV) and (X) to (XX), where bonding to the bridge B occurs via one of the radicals R2 to R19, R22 to R27, R28 to R58, R61, R62 R67 R68, R122 or, if one of the radicals E1 or E2 represents NR1 or one of the radicals E3 to E11 represents NR59 or one of the radicals E101 to E102 represents NR119, via R1, R59 or R119 and the said radicals then represent a direct bond, and
B represents a bridge of the formula xe2x80x94(CH2)nxe2x80x94 or xe2x80x94[Y1s(CH2)mxe2x80x94Y2]oxe2x80x94CH2)pxe2x80x94Y3qxe2x80x94 which may be substituted by C1- to C4-alkyl, C1- to C4-alkoxy, halogen or phenyl,
Y1 to Y3 represent, independently of one another, O, S, NR60, COO, CONH, NHCONH, cyclopentanediyl, cyclohexanediyl, phenylene or naphthylene,
R60 is C1- to C6-alkyl, C2- to C6-alkenyl, C4- to C7-cycloalkyl, C7- to C15-aralkyl or C6- to C10-aryl,
n is an integer from 1 to 12,
m and p are each, independently of one another, an integer from 0 to 8,
o is an integer from 0 to 6 and
and s are, independently of one another, 0 or 1.
In another type of oligomeric or polymeric system, the groups OX2 and/or RED1 can also be bound, for example, as side chains to a main group, for example a poly(meth)acrylate, silicone, polycarbonate, polyurethane, polyurea, polyester, polyamide, cellulose or another oligomeric or polymeric system.
Examples of metal salts or metal complexes which can be used as OX2 or RED1 are Fe3+/2+, Ni3+/2+, Co3+/2+, Cu2+/+, [Fe(CN)6]3xe2x88x92/4xe2x88x92, Fe4[FE(CN6]30/4xe2x88x92, [Co(CN6]3xe2x88x92/4xe2x88x92, [Fe(cyclopentadienyl)2]0/+, Lu(Pc)2+ to 2xe2x88x92 (Pc=phthalocyanine), Fe[Fe(CN)6]0/1xe2x88x92.
Possible counterions for metal ions and cationic complexes are all redox-inert anions Xxe2x88x92 as described in more detail below; possible counterions for the anionic complexes are all redox-inert cations Mxe2x80x2+, for example alkali metals or quaternary ammonium salts such as Na+, K+, N(CH3)4+, N(C4H9)4+, C6H CH2N(CH3)3+ and others
Likewise preferred is an electrochromic device containing mixtures of the electrochromic substances mentioned above in general terms or as being preferred. Examples of such mixtures are (II)+(CI)+(XVI), (II)+(IV)+(XII), (Ia)+(II)+(XVI), (Ia)+(CI) without this implying any restriction.
The mixing ratios can be varied within wide limits. They allow the optimization of a desired colour shade or degree of blackness and/or the optimization of the desired dynamics of the device.
In the above definitions of substituents, alkyl radicals include radicals derived therefrom, e.g. alkoxy or aralkyl radicals, preferably those having from 1 to 12 carbon atoms, in particular from 1 to 8 carbon atoms, unless otherwise indicated. They can be linear or branched and may bear further substituents such as C1- to C4-alkoxy, fluorine, chlorine, hydroxyl, cyano, C1- to C4-alkoxycarbonyl or COOH.
Cycloalkyl radicals are preferably ones having from 3 to 7 carbon atoms, in particular 5 or 6 carbon atoms.
Alkenyl radicals are preferably ones having from 2 to 8 carbon atoms, in particular from 2 to 4 carbon atoms.
Aryl radicals, including those in aralkyl radicals, are phenyl or naphthyl radicals, in particular phenyl radicals. They can be substituted by from 1 to 3 of the following radicals: C1- to C6-alkyl, C1- to C6-alkoxy, fluoro, chloro, bromo, cyano, hydroxy, C1- to C6-alkoxycarbonyl or nitro. Two adjacent radicals may also form a ring.
For the purposes of the present invention, aromatic or pseudoaromatic, five- or six-membered heterocyclic rings which may optionally be benzo-fused are, in particular, imidazole, benzimidazole, oxazole, benzoxazole, thiazole, benzothiazole, indole, pyrazole, triazole, thiophene, isothiazole, benzoisothiazole, 1,3,4- or 1,2,4-thiadiazole, pyridine, quinoline, pyrimidine and pyrazine. They may also be substituted by from 1 to 3 of the following radicals: C1- to C6-alkyl, C1- to C6-alkoxy, fluoro, chloro, bromo, cyano, nitro, hydroxy, mono- or di-C1- to C6-alkylamino, C1- to C6-alkoxycarbonyl, C1- to C6-alkylsulphonyl, C1- to C6-alkanoylamino, phenyl or naphthyl. Two adjacent radicals can also form a ring.
The electrochromic substances are either known (Topics in Current Chemistry, Vol. 92, p. 1-44, (1980), Angew. Chem. 90, 927 (1978), Adv. Mater. 3, 225, (1991), DE-OS (German Published Specification) 3,917,323, J. Am. Chem. Soc. 117, 8528 (1995), J. C. S. Perkin 11 1990, 1777, DE-OS (German Published Specification) 4,435,211, EP-A 476,456, EP-A 476,457, DE-OS (German Published Specification) 4,007,058, J. Org. Chem. 57, 1849 (1992) and J. Am. Chem. Soc. 99, 6120, 6122 (1977) or can be prepared by analogous methods. The compounds of the formula (I) are likewise known (WO 97/30134) or can be synthesized from building blocks known per se, for example as shown in the following scheme: 
Ions such as bromide formed as a result of the synthetic method are subsequently replaced by redox-inert ions.
The electrochromic display device of the invention contains at least one solvent in which the electrochromic substances, if desired a conductive salt and, if desired, further additives. The solvent may also be thickened to form a gel, for example by means of polyelectrolytes, porous solids or nanosize particles having a large active surface area.
Suitable solvents are all solvents which are redox-inert under the electric potentials selected and cannot split off any electrophiles or nucleophiles or themselves react as sufficiently strong electrophiles or nucleophiles and thus be able to react with the coloured radical ions. Examples are propylene carbonate, xcex3-butyrolactone, acetonitrile, propionitrile, glutaronitrile, methylglutaronitrile, 3,3xe2x80x2-oxydipropionitrile, hydroxypropionitrile, dimethylformamide, N-methylpyrrolidone, sulpholane, 3-methylsulpholane or mixtures thereof. Preference is given to propylene carbonate and mixtures thereof with glutaronitrile or 3-methylsulpholane.
The electrochromic solution used for the purposes of the invention can contain at least one inert conductive salt. If, in particular, at least one of the substances of the redox pair RED1/OX2 is ionic in nature, the addition of a conductive salt can be omitted.
Suitable inert conductive salts are lithium, sodium and tetraalkylammonium salts, in particular the latter. The alkyl groups may have from 1 to 18 carbon atoms and be identical or different. Preference is given to tetrabutylammonium. Anions which are suitable for these salts, and also suitable as anions Xxe2x88x92 in the formulae (II) to (VI), (CI), (CII) and (CV) to (CVII) in in the metal salts are all redox-inert, colourless anions.
Examples are tetrafluoroborate, tetraphenylborate, cyanotriphenylborate, tetramethoxyborate, tetrapropoxyborate, tetraphenoxyborate, perchlorate, chloride, nitrate, sulphate, phosphate, methanesulphonate, ethanesulphonate, tetradecanesulphonate, pentadecanesulphonate, trifluoromethanesulphonate, perfluorobutanesulphonate, perfluorooctanesulphonate, benzenesulphonate, chlorobenzenesulphonate, toluenesulphonate, butylbenzenesulphonate, tert-butylbenzenesulphonate, dodecylbenzenesulphonate, trifluoromethylbenzenesulphonate, hexafluorophosphate, hexafluoroarsenate, hexafluorosilicate, 7,8- or 7,9-dicarbanido-undecaborate(xe2x88x921) or (xe2x88x922), which may bear one or two methyl, ethyl, butyl or phenyl groups as substituents on the B and/or C atoms, dodecahydro-dicarbadodecaborate(xe2x88x922) or B-methyl-C-phenyl-dodecahydrodicarbadodecaborate(xe2x88x921).
The conductive salts are preferably used in the range from 0 to 1 mol/l.
As further additives, it is possible to use thickeners in order to control the viscosity of the electroactive solution. This can be important for avoiding segregation, i.e. the formation of streaky or spotty colour on prolonged operation of the electrochromic device in the switched-on state, and to control the decolouration rate after switching off the power.
Suitable thickeners are all compounds customary for this purpose, e.g. polyacrylate, polymethacrylate (Luctite L(copyright)), polycarbonate or polyurethane.
Further possible additives for the electrochromic liquid are UV absorbers. Examples are UVINOL(copyright) 3000 (2,4- dihydroxybenzophenone, BASF), SANDUVOR(copyright) 3035 (2-hydroxy-4-n-octyloxybenzophenone, Clariant), Tinuvin(copyright) 571 (2-(2H-benzotriazol-2-yl)xe2x88x926-dodecyl-4- methylphenol, Ciba), Cyasorb 24(trademark) (2,2xe2x80x2-dihydroxy-4-methoxybenzophenone, American Cyanamid Company), UVINUL(copyright) 3035 (ethyl 2-cyanoxe2x88x923,3-diphenylacrylate, BASF), UVINUL(copyright) 3039 (2-ethylhexyl 2-cyano-3,3-diphenylacrylate, BASF), UVINUL(copyright) 3088 (2-ethylhexyl p-methoxycinnamate, BASF), CHIMASSORB(copyright) 90 (2-hydroxy-4- methoxybenzophenone, Ciba).
The last four examples mentioned are preferred. Likewise preferred are mixtures of UV absorbers, for example the last four examples mentioned. Preference is given to the mixture of UVINUL(copyright) 3039 (BASF) and CHIMASSORB(copyright) 90.
The UV absorbers are used in the range from 0.01 to 2 mol/l, preferably from 0.04 to 1 mol/l.
In the electrochromic solution, the electrochromic substances OX2 and REd1, in particular those of the formulae (I) to (XX) and (CI) to (CIV), are each present in a concentration of at least 10xe2x88x924 mol/l, preferably from 0.001 to 0.5 mol/l. The total concentration of all electrochromic substances present is preferably less than 1 mol/l.
To operate the electrochromic display device of the invention, a constant, pulsed or variable-amplitude DC potential, for example a variable sinusoidal, rectangular or triangular DC potential is used.
However, it is also possible to use an AC potential, i.e. a potential whose polarity changes at a certain frequency. This voltage change can be rectangular, triangular, sinusoidal or of any other shape. In particular, the phases of opposite polarity can have different lengths.
The frequency of the AC potential or the pulsed or variable-amplitude DC potential can be in the range from 10xe2x88x922 to 104 Hz, preferably from 10xe2x88x921 to 103 Hz, particularly preferably from 10 to 5xc3x97102 Hz. The frequency can also be alterable during operation. A particularly preferred shape of the variable-frequency AC potential is the rectangular AC potential shown in FIG. 1 and also modifications thereof having the same frequency sequence but a triangular or sinusoidal shape.
The amplitude of the applied voltage depends on the desired depth of colour and on the reduction and oxidation potentials of the OX2 and RED1 used. Such potentials may be found in Topics in Current Chemistry, Volume 92, p. 1-44, (1980) or Angew. Chem. 90, 927 (1978) or the literature cited therein. The difference between the reduction and oxidation potentials provides a guideline for the required voltage, but the electrochromic display device can be operated at a lower or higher voltage. In many cases, e.g. when using OX2=formula (II) or (IV) and RED1=formula (X), (XII), (XVI) or (XVII) or when they are linked via a bridge as in formula (I), in particular formulae (Ia) to (Id), the voltage required for operation is xe2x89xa61 V. Such electrochromic display devices can be supplied in a simple way with power from photovoltaic silicon cells.
When the electric potential is switched off, the electrochromic display device of the invention loses its colour again. This extinguishing of the colour can be accelerated by the segments or plates to which power is supplied being short-circuited. The display can also be extinguished very rapidly by repeatedly reversing the polarity of the applied potential, if desired while simultaneously reducing the potential.
The switching-on and switching-off times of the display device can be influenced within wide limits by variation of the layer thickness of the electrochromic display device, the viscosity of the electrochromic solution and/or by the selection of electrochromic substances with regard to their diffusion or drift behaviour. Thus, for example, thin layers have shorter switching times than thick ones. The size of the molecules of the electrochromic medium, in particular in the case of the systems bridged via B or the oligomeric or polymeric systems, influences the diffusion or drift behaviour. The larger the molecules, the lower their diffusion and drift. The charge on the molecules also influences the drift behaviour. The higher the charge on a molecule of given size, the greater the drift rate. It is thus possible to construct display devices which switch quickly and slowly and are optimally matched to the respective application.
The display device can be operated in a power-saving or refresh mode. In the power-saving or refresh mode, the DC potential or AC potential applied to the display device is interrupted every now and again. In the phases during which no voltage is applied, the contacts of the display device are not electrically connected. The phases during which voltage is applied and the phases during which no voltage is applied alternate and can be of identical or different duration. In a preferred operating mode, the phases during which the voltage is applied are shorter than the phases in which no voltage is applied. The ratio can be from 1:1.5 to 1:30, preferably from 1:2 to 1:10. The absolute duration of the phases can be very different and depends essentially on the construction of the display device. As the viscosity of the electrochromic medium and/or layer thicknesses of the display device increase, the absolute duration of the phases can increase. At layer thicknesses of from 100 to 400 xcexcm, the phase duration can be in the range from 0.5 to 20 seconds. In the case of a low-viscosity electrochromic medium and/or when the layer thicknesses of the display device are small, e.g. from 5 to 50 xcexcm, the absolute duration can be less than 1 second, for example from 0.001 to 0.5 second, preferably from 0.01 to 0.1 second. The phases in which no voltage is applied and therefore no current flows result, depending on the ratio of the phase lengths, in considerable savings of electric power. At a ratio of 1:9, for example, the power saving is 90% compared with continuous operation. Fluctuation or flickering of the intensity of the switched-on electrochromic display device or its segments or pixels and fuzziness of the segments or pixels caused by diffusion during the phases during which no voltage is applied are avoided by means of the construction of the display device and the absolute phase length matched thereto.
The display devices of the invention can be used as segmented or matrix displays in watches, clocks, computers, electric appliances, electronic appliances such as radios, amplifiers, TVs, CD players, in destination displays in buses and trains, in departure or arrival boards in railways stations and airports, in flat VDUs, in status displays such as indicators for the charging state of power sources, fill level displays or speed indicators.
Further applications are screens or panes such as window panes, partitions or eye-protection screens in offices, road vehicles, aircraft, trains, bank counters, door glazing, motorcycle or pilots"" helmets, glazing of household appliances, mirrors of any type such as flat, spherical or aspherical or combinations thereof for road vehicles and trains, provided that these contain at least one switchable, static or variable display device. Possible displays in partitions can bexe2x80x9cPlease do not disturbxe2x80x9d or xe2x80x9cPosition closedxe2x80x9d. In automobile mirrors, it is possible to display, for example, the outside temperature, the time, the compass bearing or malfunctions in the vehicle, for example information regarding the oil temperature or open doors.